1. Field of the Invention
The present invention relates to a protective sheet for recording sheets and a printer in use therewith. More particularly, the present invention relates to an improvement in which a protective sheet used with plural recording sheets is automatically treated.
2. Description Related to the Prior Art
A thermal printer is used with sheets of thermosensitive recording material, which includes a support and at least one recording layer formed on the support. The thermal printer has a thermal head, with which an image is thermally recorded on the recording sheet. Ultraviolet rays of a predetermined range of wavelength are applied to the recording layer, to destroy its coloring ability, as the recording layer has optical fixability responsive to the ultraviolet rays of the limited range. If the recording sheet is exposed to light from fluorescent lamps generally used in a room for illumination, the fluorescent light influences the coloring ability of the sheet. It is usual to stack a plurality of recording sheets and wrap them in a light-shielding wrapper, bag or box, to be supplied to a user.
To load the thermal printer with the recording sheets, the light-shielding wrapper is opened first. The stacked recording sheets are taken out of the wrapper. It is inevitable that a recording surface of only one of the stacked recording sheets remains uncovered, and is exposed to ambient light including fluorescent light. To protect the coloring ability of the recording surface from influences, it is proposed to overlap a protective sheet on the uncovered recording surface for the purpose of shielding light.
It is known, as disclosed in JP-A (Japanese Patent Laid-open Publication No.) 1-249493, to use a protective sheet in a thermal printer which is of a thermal transfer type used with an ink sheet. The protective sheet is for the purpose of protecting recording surfaces from fingerprints or dust. This thermal printer has a paper sensor of a reflecting type disposed above a sheet cassette for containing the sheets to be fed. When all the recording sheets are fed, only the protective sheet remains in the sheet cassette. The protective sheet has a code printed thereon for inhibiting feeding. The paper sensor detects the inhibiting code on the protective sheet. This provides the printer with information of lack of recording sheets. The printer stops feeding the sheet.
The thermal printer can have a clamper which fixes the recording sheet on the platen drum for the thermal recording. This printer would require a home position detecting sensor for detecting a state where the clamper stands by near an end of a sheet feeding path, and a sheet advance detecting sensor for detecting whether an advancing edge of the recording sheet has come to the clamper. In addition, the platen drum might be provided with a sensor for detecting a releasing position, and a sensor for detecting a sheet being exited, for the purpose of ensuring exiting of the sheet. If the paper sensor of the above document is disposed in association with the sheet cassette, this may raise the cost of the printer, as wires connecting the relevant circuits may be complicated. Steps of assembling the printer may be enlarged.